1. Field of the Invention
This invention relates to apparatus for controlling the movement of a fluid relative to a container body in which it is stored and especially to a controllable fluid damper for a fluid-containing tank of a space satellite.
2. Description of the Prior Art
Positive-expulsion fuel and oxidizer tanks such as those used in space satellites, for example, have such severe design and operational problems that the current trend is toward use of free surface tanks. Free surface tanks, however, have two very significant problems. The first is one of pressurization gas flow into the propellant feed line. This problem can apparently be controlled by the use of a propellant acquisition system using surface tension screens. The second and potentially more serious problem, involves loss of control of the liquid bulk, and interaction of the fluid, structure and stage control system.
During a stage maneuver in space the fluid is set into motion by pressure forces normal to the tank surface and also by viscous shear forces due to relative motion between the fluid and tank wall. After the stage maneuver the fluid remains in motion until its motion relative to the tank wall can be damped. The fluid motion is primarily rotation, flowing around the inner surface, circulating through screens and floating across the tank. With no damping, forces are still applied to the tank due to the curved paths of the fluid. In addition, viscous forces due to relative motion between the fluid and tank act on the tank as well as the fluid. The net effect of the torques and forces on the stage orientation must be counteracted by torques commanded by the control system.
Designs using conventional baffles increase the coupling between the fluid and tank so that the rate of decay of the fluid momentum is increased. During the transient maneuver, however, the coupling is also increased so that more fluid is involved in the stage motion. Just after the maneuver then, the residual torques of the fluid on the tank will be higher with the baffled tank than tank without baffles. Due to the difference in decay rate, there will be a time where torques for both types of tank are about equal. For greater time, the baffled tank will show lower torques.